With an increase in speed and integration of semiconductor integrated circuits, there is a problem of electromagnetic interference (EMI). In order to reduce the EMI, an increasing number of devices operate with spread-spectrum processed clock signals.
The spread-spectrum processed clock signals can be obtained by applying specific controls for spread spectrums on phase locked loops (PLLs), which generate clock signals with desired frequencies. Traditionally, an input voltage to a voltage-controlled oscillator (or an input current when a current-controlled oscillator is used) in a PLL has been modulated, or a dividing ratio of a frequency divider has been changed to generate a spread-spectrum processed clock signal (see, e.g., Patent Documents 1 and 2). Another conventional approach has been to accordingly select and output one of a plurality of clock signals output from a voltage-controlled oscillator, and having different phases or frequencies (see, e.g., Patent Document 3).